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DCV, VOCs and moisture set points in a multifamily building!

A traditional mechanical ventilation system with heat recovery (MVHR) for multifamily houses runs on a constant air volume (CAV) principle. A problem, is that a CAV-system will ventilate unnecessarily much at times and insufficiently at other times. A novel MVHR has been developed to address this issue [3]. This DCV-based system regulates the ventilation rate for each apartment according to two parameters in the indoor air: the moisture supply, and the content of volatile organic compounds (VOCs). Outside each apartment, a box is placed that includes sensors and dampers that regulate the ACR for the apartment based on the measured parameters. The moisture supply, which is the difference between the indoor and outdoor vapor content, is calculated through measurements of the vapor content in the supply and in the exhaust air. The idea is to ventilate only as much as necessary in order to achieve a healthy indoor environment and at the same time reduce the energy output.

An important factor that affects our health is the state of the building and the building materials. For this, and other reasons, we must strive to avoid moisture damage to the building materials by controlling the moisture levels in our houses. It is therefore important to carefully analyze the ventilation strategy, which includes the set points for the ventilation system. In this novel DCV-system, the ventilation rate increases when the measured moisture supply exceeds the set point based on a PI-controller. Shortly after, the ventilation rate reaches its maximum. As long as the set point is exceeded the ventilation system strives to reduce the moisture supply until it reaches below the set point. When the moisture supply set point has not been exceeded, the ventilation rate is adjusted to deal with the VOCs instead. In the case of a too high set point for the moisture supply, the risk for mold growth in the building material increases. In the case of a too low set point, the system’s energy efficiency might be decreased since the system would sometimes ventilate at an unnecessarily high rate, i.e. when the risk for moisture damage is insignificant.

Today, there are no directives from the authorities in Sweden concerning the moisture supply, and we have not found research that suggests a limit on the moisture supply. There are investigations [4] of the indoor environment in the Swedish multifamily buildings that were built between the years of 1965-1975; the so called “million program”. These investigations show that the average of the moisture supply in these buildings is approximately 1.2 g/m3 with a ventilation rate of about 0.5 ac/h. However, these investigations do not address the risk of moisture damage in building materials due to the moisture supply.

In a previous article we have evaluated the set points for the VOC-control and the moisture supply for this DCV, and suggested an adjustment [3]. However, the moisture supply set point was suggested based on the results of the above mentioned investigations [4], and not on the risk for moisture damage. The study presented here has analyzed the most vulnerable situation where the moisture supply is the key factor for moisture damage in the concerned buildings. This, in order to determine a set point that is to be used when implementing this kind of DCV-system.